The compounds of the formula: ##STR1## wherein R.sub.1 and R.sub.2 are, independent from each other, hydrogen, alkyl, or cyclic or bicyclic alkyl; R.sub.3 is an acyl substituent selected from the group consisting of formyl, alkanoyl, alkenoyl, alkylsulfonyl, aroyl, arylalkenoyl, arylsulfonyl, arylalkanoyl or arylalkylsulfonyl; A is selected from the group consisting of: ##STR2## wherein R.sub.4 is hydrogen, alkyl, perfluoroalkyl, alkoxy, perfluoroalkoxy, amino, alkylamino, dialkylamino, alkylsulfonamido, alkylcarboxamido, nitro, cyano or carboxyl; or a pharmaceutically acceptable salt thereof, are smooth muscle relaxants.